The invention also concerns a shift control method in an automated transmission of a motor vehicle, whose input shaft can be connected by means of a separator clutch in the form of an automated friction clutch to the driveshaft of a drive engine, whose output shaft is in driving connection with the axle transmission of a drive axle or with a transfer box, and whose starting gears can be engaged by means of gear clutches in the form of unsynchronized claw clutches, wherein a tooth-on-tooth position during a correction shift from an engaged first starting gear to a second starting gear that is to be engaged, which position occurs at the associated gear clutch when the second starting gear is being engaged while the vehicle is at rest, is resolved as comfortably as possible.
Automated transmissions, particularly those used in commercial vehicles, are often equipped with gear clutches in the form of unsynchronized claw clutches. Compared with friction-synchronized gear clutches provided with blocking teeth, namely so-termed friction-synchronized gear clutches, unsynchronized claw clutches have an essentially more simple and more compact structure and are substantially more robust in driving operation. Thus, transmissions equipped with unsynchronized claw clutches can, on the one hand, be produced less expensively, and, on the other hand, have a longer service life than transmissions provided with friction-synchronized gear clutches.
In order to be able to engage unsynchronized claw clutches, the respective halves of the clutch, usually consisting of a clutch ring gear with outer teeth and a shifting sleeve with inner teeth, have to be brought to approximately the same rotational speed. The clutch ring gear is arranged laterally either on a loose wheel of a spur gear stage or on a hollow shaft section of a transmission component such as a sun gear, a planetary carrier or a ring gear of a planetary gearset. The shifting sleeve is arranged rotationally fixed but able to move axially on that transmission shaft on which the loose wheel of the spur gear stage concerned or the hollow shaft section of the planetary gearset concerned, is mounted to rotate.
When a claw clutch is engaged, however, which takes place by a corresponding axial displacement of the shifting sleeve, a so-termed tooth-on-tooth position can occur in which the front sides of the teeth of the shifting sleeve butt up or come in contact against the front sides of the teeth of the clutch ring gear, whereby the meshing of the shifting sleeve teeth in the tooth gaps of the clutch ring gear, and hence the engagement of the gear clutch concerned is prevented or at least delayed. In a gearshift during driving, a tooth-on-tooth position occurs relatively seldom because the two halves of the clutch, by virtue of their respective driving connections to the input shaft or the output shaft, are usually being driven at different rotational speeds. In contrast, since when the vehicle is at rest the input and output shafts are not moving, when a starting gear is being engaged a tooth-on-tooth position occurs more often. This is particularly the case when, with the vehicle at rest, a correction shift from an engaged first starting gear to a second starting gear to be engaged is carried out, since when a tooth-on-tooth position exists at the gear clutch of the first starting gear a tooth-on-tooth position at the gear clutch of the second starting gear is all the more probable because of the different transmission ratios of the starting gears. Such a correction shift is needed, for example, when a first starting gear has already been engaged in a commercial vehicle for driving up a loading ramp, and then, as a result of loading or unloading, the vehicle mass changes so substantially that the engaged starting gear is no longer suitable for the forthcoming starting process. Consequently, the transmission ratio of a second starting gear can be higher or lower than the transmission ratio of a first starting gear.
To resolve a tooth-on-tooth position at a claw clutch, rotation of the input-side clutch half relative to the output-side clutch half is required. For this, it is generally known to briefly act upon the input shaft in driving connection with the input-side clutch half with a drive torque or a braking torque. As is known, this can be done by partially closing the separator clutch, or by engaging an auxiliary motor that is in driving connection with the input shaft, or by engaging a transmission brake that is in driving connection with the input shaft.
U.S. Pat. No. 6,769,523 B2 describes a method for resolving a tooth-on-tooth position produced by a gearshift at a gear clutch in the form of a claw clutch, in accordance with which the starting and shifting clutch is closed farther in steps until the gear clutch concerned is engaged, or until a discontinuation criterion is fulfilled.
In contrast, in a method known from DE 10 2006 046 605 A1 it is provided that at every gearshift the starting and shifting clutch is partially closed and the drive torque imposed thereby on the input shaft is compensated by a transmission brake in driving connection with the input shaft. When a shift-related tooth-on-tooth position at a claw clutch occurs, the input shaft is then accelerated or slowed down by varying the braking torque of the transmission brake.
From DE 10 2008 054 635 A1 a method is known, in which a shift-related tooth-on-tooth position at a claw clutch is resolved by virtue of an electric machine in driving connection with the input shaft, by rotating the input-side half of the clutch through a defined rotational angle.
Finally, DE 10 2011 087 376 A1 describes a transmission control device for an automated transmission, which is designed such that a shift-related tooth-on-tooth position at a claw clutch is resolved either rapidly but with little comfort, or slowly but more comfortably, depending on at least one operating condition.
In the known method the gear clutch concerned is often engaged after the tooth-on-tooth position has been resolved while the input-side half of the clutch is still rotating, and therefore under the action of the drive torque that is producing the rotation of the input-side clutch half. This results in an abrupt drop of the drive torque, which is expressed as comfort-reducing shift jerk and a sudden loading of the drive-train as a whole. The shift jerk is particularly perceptible when the vehicle is at rest, i.e. when a starting gear is engaged or during a correction shift from an engaged first starting gear to a second starting gear that is to be engaged, and it then also gives rise to a starting jerk that has to be cushioned by timely engagement of the parking brake or by actuating the service brake of the vehicle.